Display devices with differing images on each side and being transparent or translucent from one of the sides are known from a variety of documents including EP-A-0170472 which describes a panel comprising a light permeable material and a silhouette pattern, comprising any arrangement of light restricting material which subdivides the panel into a plurality of discrete light restricting areas and/or a plurality of discrete transparent or translucent areas, characterized in that a design is superimposed on or forms part of said silhouette pattern so that said design is visible from one side of the panel only, and wherein said design is less perceptible from said one side of the panel as the level of illumination transmitted through the panel from said other side increases. A number of different vision effects are obtainable from different panels falling within the above definition. Thus clarity of vision can be maintained from the one side to the other side with the exception of the area covered by the design with clarity of vision through the whole of the panel from the other side to the one side. Visibility from the one side to the other side can be totally or partially obstructed while there is clarity of vision through the whole of the panel from the other side to the one side, in other words a unidirectional vision effect is obtained. Clarity of vision is obtainable from the one side to the other side except in the area of the design while visibility from the other side to the one side is totally or partially obstructed. Vision from either side can be totally or partially obstructed. In all cases through vision can be obtained in either direction through the panel when the level of illumination perceived through the panel from the far side of the panel sufficiently exceeds the illumination reflected from the near side of the panel. The transparent areas typically have dimensions ranging from 0.5 to 3 mm.
EP-A-0170472 and EP-00118638 describe methods of producing both the silhouette pattern and also the imposed design. The methods as described may be summarized as either sequential printing of the silhouette and/or the design using screen lithographic or similar ink printing processes with as exact a registration as can be obtained or a method in which a mask is applied and the printing processes are carried out through the mask onto the substrate. When the mask is removed, the silhouette pattern and image remain on the substrate only in the areas which the mask or stencil allow the ink to penetrate.
EP-A-0234121 describes further methods of printing such an image. The printing methods are limited to those including inks. Again a mask is described which is subsequently removed taking with it unwanted portions of the silhouette pattern and image.
U.S. Pat. No. 5,396,559 describes a security device for use on identification cards, monetary documents, and the like using a reference pattern and a message pattern each having the appearance of a random pattern of dots. The reference pattern is a dense pattern of randomly positioned dots, and the message pattern is a modulated version of the reference pattern in which the dots of the reference pattern are slightly repositioned by an amount depending on the gray value or color value of a message image at each dot location. The message image is decrypted and becomes visible with a range of gray values when it is viewed through a film transparency of the reference pattern. The dot pattern may be printed, embossed or recorded as a photograph or a hologram. Decryption of the message image may be accomplished by viewing through a contact mask, superposition of images of the message pattern and reference pattern, by viewing the message pattern through a mask positioned at a real image of the reference pattern, or like means.
Japanese patent application Kokai I (1993)-57863 describes a production of an image including transparent sections for areas of the image. A method is described in which a decorative sheet is prepared by registration printing in the order of a rear pattern layer, a covering ink layer and a front pattern layer on a transparent plastic sheet in such manner that a plurality of small transparent portions remain in the image. No description is made as to how the registration printing should be carried out.
Japanese patent application Kokai I (1989)-69397 describes a method of producing a transparent plastic or glass substrate with a printed layer including a plurality of holes. The method includes printing the image onto a second substrate, perforating the image and second substrate and then transferring the image only from the second substrate to the transparent plastic or glass substrate.